Pickling process



Patented June 20,- 1950 PICKLING raocass Henry Francis Myers, Pittsburg, and Otto Albert Proeiss, Antioch, CaliL, assignors to Columbia Steel Company, a corporation of Delaware No Drawing. Application May 9, 1945,

Serial No. 592,924

(Claims.

This invention relates to an improved process of pickling ferrous articles to remove scale, rust, and other foreign matter therefrom and to prepare their surfaces for the reception of an adherent coating.

The invention has among its objects the provision of an improved method of pickling ferrous articles in preparation for the reception of an adherent coating, for example a galvanized coating, whereby both efficient scale removal and suitable etching of the article are efiected without undue attack of the metal or the formation of substantial amounts of nascent hydrogen.

A further object of the invention lies in the provision of a method of surface treatment of the ferrous articles preparatory to coating the same to provide cleaner surfaces, free from both onion skin and flashing, and a brighter, more adherent and more uniform coating, such coating in the case of galvanizing being better spangled than formerly.

A still further object is the provision of a method of surface treatment of ferrous articles such that, upon their being galvanized, less dross is formed in the galvanizing pot. 1

These and further objects of the invention will become more apparent in the following description.

In the manufacture of coated metallic products such as galvanized steel sheets, it is necessary to clean them to remove scale, oxides, or other foreign matter which will prevent a satisfactory bond between the metal base and the coating. In preparing steel sheets for galvanizing, such cleaning is usually accomplished by pickling in a solution of sulphuric acid, usually ranging in concentration from 5 to 10% by weight of 98% sulphuric acid and maintained at a temperature of from 150 to 180 F. The time of immersion will vary depending upon the type of steel and the adherence of coating required. After thorough cleaning by this method the sheets are washed and transferred to the galvanizing equipment where they are usually given a short dip in hydrochloric acid of approximately 10% concentration from which they pass through a molten flux of ammonium chloride and zinc-ammonium chloride into the zinc spelter.

Unless some inhibiting agent is present, hydrogen will be liberated by the attack of the acid on the steel. The nascent hydrogen is readily occluded by the steel and may cause blisters therein if it is liberated to form molecular hydrogen at some void, lamination, or inclusion in the interior of the steel. In order to prevent or minimize this tendency, it is customary to use an inhibiting agent of some type in the pickling solution which will prevent or minimize the attack on the steel and the consequent absorption of nascent hydrogen. On the other hand, in

order to obtain a good bond between the coating and the steel base, it is necessary to etch the sheet during pickling. This etching is accomplished by permitting the acid to erode a certain proportion of the steel surface. In the conventional pickling process, therefore, the amount of inhibitor is adjusted with the acid concentration, temperature, and time of immersion so that the required etching will be obtained, if possible, without blistering the sheets. If a particular steel is susceptible to blistering, it is impossible to etch it sufficiently to obtain a tight coat, that is, an adherent bond, without the production of an excessive amount of blisters. Furthermore, a completely satisfactory etching cannot be obtained with the use of an inhibitor due to the fact that the inhibitor prevents or minimizes the attack on the steel base by the acid.

The present invention provides a method of pickling and etching which overcomes the inability to obtain a satisfactory etching in the pickling solution to which an inhibitor has been added, while still maintaining the advantages of a substantially or fully inhibited pickling bath to prevent excessive attack of the steel by the acid during removal of the scale, and thus to prevent the formation of nascent hydrogen which may cause blistering. Briefly, the invention involves pickling the ferrous articles, such as sheets, in an inhibited sulphuric acid solution for only a suflicient time to remove the hot rolling and/or annealing oxide, and the subsequent pickling, after washing, in an uninhibited sulphuric acid solution to which a small amount of a strongly oxidizing material has been added to activate the pickling acid and to destroy or markedly decrease the inhibiting properties of the inhibitor which is inevitably carried over into the pickle bath by adherence to the surfaces of the articles being pickled.

Specifically the process, which may be employed on any ferrous articles to be coated as by galvanizing, but which is here illustrated in connection with steel sheets, consists in immersing such articles in a first sulphuric acid pickling bath containing from 6 to 8% by weight of 98% sulphuric acid at a temperature of from to 185 F., preferably between and F., for a period of from 30 to 45 minutes, depending upon the amount of scale, rust, and the like and 3 its degree of adherence to the base metal. Such first pickling bath contains a small quantity of an organic inhibiting material serving substantially or-wholly to prevent attack of the base metal by the pickling acid. Among the inhibitors which may be used are those known as Rodine No. 10''! and Rodine No. 111, manufactured by the American Chemical Paint Company, Ambler, Pennsylvania, both of which contain dlorthotolylthiourea as the active ingredient. Such inhibitor is added to a fresh pickle solution within the range of 4 to 16 ounces of inhibitor per 1300 galions of solution, the higher quantities of inhibitor being employed with the lighter gauges of metal being pickled. Additions to the solution of from 1 to 5 ounces of inhibitor for each rack of sheets pickled are made after the pickling of predetermined numbers of racks of sheets to maintain the inhibitor concentration within the initial range.

Following such first pickling step the metal being treated is thoroughly rinsed in water for a period which usually ranges from 2 to 5 minutes, following which the metal is immersed in a second pickling bath in order to etch the surface to the desired extent so that the coating subsequently to be applied will adhere tightly thereto. The second pickling bath consists of a sulphuric acid solution containing from 6 to 8% by weight of 98 sulphuric acid with the addition of from .50 to 1% by Weight of the solution of an oxidizing material which activates the pickling acid. It has been found that materials yielding a nitrate radical upon ionization are ideal for this purpose. Among such materials are nitric acid, ammonum nitrate, sodium nitrate, and potassium nitrate. The second pickling bath is maintained at a temperature between 150 to 165 .5 and the material is left in the solution for periods ranging from 10 to minutes, the lower temperature and shorter pickling time being employed with sheets of lighter gauge.

The procedures outlined in the following table are typical of the two-step pickling method of the present invention as applied to flat galvanized sheets of from 11 to gauge. It is to be understood, however, that the invention may be employed with sheets of other types, such as corruing scale and etching the base metal. Sheets treated by our method are particularly characterized by their freedom from onion skin" and flashing, which are the result of the reduction of scale, which is essentially iron oxide, in the annealing process so that a layer of reduced iron is present on the surface of the sheet. This reduced iron is very resistant to attack by sulphuric acid and as a result cannot readily be removed if an inhibitor is present in the acid solution. Thus, when a sheet pickled in the conventional manner in a partially inhibited solution is introduced into the galvanizing pot the reduced iron on its surface, which such pickling solution has failed to remove, is detached, forms an iron zinc alloy with the molten zinc in the pot and results in an increased amount of dross.

The use of our two-step pickling process, on the other hand, insures the removal of the layer of reduced iron from the surface of the sheets in the second pickling solution, thereby eliminating onion skin, flashing, and excessive dross in the galvanizing pot. In addition, the activated uninhibited second pickling solution allows uniform etching of all parts of the sheet without over-pickling any portions which may have been less heavily coated with scale, because the scale is completely removed by the partially or fully inhibited first pickle without any appreciable etching of the sheet. As a result, the time required in the inhibited first pickling solution for the removal of oxide or scale reduces by that amount the time the sheet or article is exposed to the absorption of the nascent hydrogen.

I more of the following actions: activation of the gated sheets, sheets to which coatings of; different i pickling acid, reduction of the over-voltage for hydrogen evolution on the surface of the article, anddestruction or marked decrease in the inhibiting properties of the inhibitors which tend to remain upon the surfaces of the articles pickled in the first solution despite the water rinse. Experimental evidence appears to substantiate the theory that the prime function of the oxidizing activator in the second solution is the destruction of the film of inhibitor on the articles carried over from the first pickle solution, thereby allowing the second solution uniformly to etch the surface of the articles. It will be understood, however, that the method of the invention is not to be limited to such theory of its action, but that its scope is rather to be defined in the appended claims,

Table Inhibited Pickle (IO-8.0% Supplemental Pickle (6.0-

Acid) 8.0% Acid) Water Gauge Rinse, 'lfime,

Time, Temp., Rodine No. Mmutes 'lemp.,

Min. F. in Oz. F.

Aim Ranges Having thus fully disclosed the novel pickling process of the present invention and typical manners in which it may be'carried out, we wish to claim the following.

We claim: a

1. The process of preparing ferrous articles for pickling solution for the dual purpose of remov- 78 galvanizing which comprises subjecting the artia,s11,osa

clesto the action of a pickling bath of dilute sulphuric acid containing an organic inhibitor to remove scale, oxides, and other foreign matter from the surfaces of the articles without substantially attacking the metal thereof, rinsing the pickled articles, and then subjecting the pickled and rinsed articles to the action of a .bath of dilute sulphuric acid containing a small amount metal, rinsing the pickled articles, and then subjecting the. pickled and rinsed articles to the action of a bath of 6 to 8% by weight of 98% sul- 1 phuric acid in water, the bath containing 0.5 to

' 1.0 by weight of an oxidizing agent selected from the class consisting of nitric acid, ammonium nitrate, and alkali metal nitrates, at a temperature of 150-165" F. to uniformly etch the surfaces of the articles to the desired degree.

3. The process of preparing ferrous articles for. galvanizing which comprises subjecting the articles to the action of a pickling bath of dilute sulphuric acid containing an organic inhibitor at a I temperature of 145-185" F. to remove scale, ox-

ides, and other foreign matter from the surfaces thereof without substantially attacking the metal, rinsing the pickled articles, and then subjecting the pickled and rinsed articles to the action of a bath of 6 to 8% by weight of 98% sulphuric acid in water, the bathcontaining 0.5 to 1.0% by weight of an oxidizing agent selected from the class consisting of nitric acid, ammonium nitrate, and alkali metal nitrates, at a temperature between 150 F. and 165 F; for a period.

of from 10 to minutes so that the surfaces of the articles are uniformly etched to the desired degree. 4. The process of preparing ferrous articles for galvanizing which comprises subjecting the articles to the action of a pickling bath of from 'I to 8% by weight of 98% sulphuric acid in water, said bath containing from 4 to 16 ounces of an inhibitor comprising a thiourea per 1300 gallons of solution, at a temperature from 150 F. to 165 1'. for a period offrom 20 to minutes to remove scale, oxides, and other foreign matter from the surfaces thereof without substantially attacking the metal, rinsing the pickled articles, and then subjecting them to the action of a bath of 6 to 8% by weight of 98% sulphuric acid in water, the bath containing 0.5 to 1.0% by'weight of sodium nitrate, at a temperature between F. and F. for a period of from 10 to 30 minutes so that the surfaces of the articles are uniformly etched to the desired degree.

5. The process of preparing ferrous articles for galvanizing which comprises subjecting the articles to the action of a pickling bath of from 7 to 8% by weight of 98% sulphuric acid in water, said bath containing from 4 to 16 ounces of an inhibitor comprising diorthotolylthiourea per 1300 gallons of solution, at a temperature from 150 F. to 165 F.'for a period of from 30 to 45 minutes to remove scale, oxides, and other foreign matter from the surfaces thereof without substantially attacking the metal, rinsing the pickled articles, and then subjecting them to the action of a bath of 6 to 8% by weight of 98% sulphuric acid in water, the bath containing 0.5 to 1.0% by weight of sodium nitrate, at a temperature between 150 F. and 165 F. for a period of from 10 to 30 minutes so that the surfaces of the articles are uniformly etched to the desired degree.

HENRY FRANCIS MYERS. O'I'IO ALBERT PROELSS.

REFERENCES crrE'n The following references are of record in the file of this patent: p Y v UNITED STATES PATENTS Number Heimberger Sept. 5, 1939 

1. THE PROCESS OF PREPARING FERROUS ARTICLES FOR GALAVANIZING WHICH COMPRISES SUBJECTING THE ARTICLES TO THE ACTION OF A PICKLING BATH OF DILUTE SULPHURIC ACID CONTAINING AN ORGANIC INHIBITOR TO REMOVE SCALE, OXIDES, AND OTHER FOREIGN MATTER FROM THE SURFACES OF THE ARTICLES WITHOUT SUBSTANTIALLY ATTACKING THE METAL THEREOF, RINSING THE PICKLED ARTICLES, AND THEN SUBJECTING THE PICKLED AND RINSED ARTICLES TO THE ACTION OF A BATH OF DILUTE SULPHURIC ACID CONTAINING A SMALL AMOUNT OF A SOLUBLE STRONGLY OXIDIZING MATERIAL YIELDING A NITRATE RADICAL ON IONIZATION, TO UNIFORMLY ETCH THE SURFACES OF THE ARTICLES TO THE DESIRED DEGREE. 